The sister and ex-girlfriend of a man arrested in a fast-growing Northern California wildfire said they strongly doubt he's responsible for starting the King Fire.

Tami Criswell, of Salinas, told KSBW on Thursday that her brother, Wayne Allen Huntsman, loves nature, camping, and has always been very cautious with campfires. He often picks up other campers' trash and douses embers left behind in fire pits, she said.

Huntsman's most recent home addresses are listed in Aptos and Santa Cruz. Criswell said her brother moved from Santa Cruz to Pollock Pines a year ago, and he works odd jobs in construction and security.

Criswell said Huntsman called her on Saturday, the same day the King Fire ignited. She said everything sounded fine over the phone. Huntsman often called his sister to check on his nieces and nephews, Criswell said.

Criswell said if her 37-year-old brother was involved in the fire, it definitely was not intentional.

"He's a really good guy," Criswell said. "He would never do anything intentionally to hurt anybody. He's always loved the wilderness."

Authorities in El Dorado County said they arrested Huntsman Wednesday and charged him with one count of arson of forest land for "willfully and maliciously" igniting the King Fire.

Huntsman's ex-girlfriend, who gave her first name as Jessica and said they dating several years, also does not believe he would deliberately spark a forest fire.

Huntsman loves nature even more than people, and he retreats into the wilderness for peace of mind, she said.

"He respected nature more than he respected people," his ex-girlfriend told KSBW.

District Attorney Vern Pierson declined to say what led investigators to Huntsman, who was scheduled to be arraigned Friday. He also would not comment on a possible motive in the case, saying the investigation was ongoing.

Wind-whipped flames burned through 76,000 acres and was 10 percent contained Thursday night, according to CalFire. It doubled in size Wednesday night and threatened more than 2,000 homes, mostly in Pollock Pines.

"It is extreme fire behavior," Fire Capt. Michelle Eidam said. "All bets are off right now because this fire is so volatile."

Those near the fire said it was powerful and dangerous. Nearly 4,500 firefighters were battling the blaze, which was threatening 12,000 homes.

"There are a lot of firefighters saying that this fire is producing fire conditions unlike anything that they have ever seen," Cal Fire Battalion Chief Joe Tyler said at a community meeting Thursday night. "It's creating its own weather overhead."

The King Fire is costing $5 million a day to fight, fire officials said.

Huntsman's bail was set at $10 million and he will be arraigned in court Friday.

Huntsman also faces a special allegation of arson with aggravating factors because the King Fire put a dozen firefighters in serious danger, forcing them to deploy their fire shields. They all escaped unharmed.